1. Field of the Invention
The present invention relates to a method of and an apparatus for manufacturing a top plate for a metallic drum container, the top plate having an opening defined therein for introducing a material into or removing a material out of the metal drum container.
2. Description of the Prior Art
Generally, metallic drum containers comprise a cylindrical drum, a disk-shaped bottom plate closing the bottom of the cylindrical drum, and a disk-shaped top plate closing the top of the cylindrical drum.
FIG. 6 of the accompanying drawings shows one conventional top plate 50 for a metallic drum container, the top plate 50 having an opening 51 defined therein for introducing a material into or removing a material out of the metal drum container. The top plate 50 has a tubular flange 52 disposed around the peripheral edge of the opening 51 and extending upwardly therefrom.
A cylindrical attachment 53 is inserted in the tubular flange 52, and has an internally threaded inner wall surface 54. The cylindrical attachment 53 serves to receive therein a plug (not shown) for closing the opening 51. The plug has an externally threaded outer wall surface, which is threaded in the internally threaded inner wall surface 54 of the cylindrical attachment 53, thereby closing the opening 51.
To prevent the contents from leaking out of the metallic drum container, it is necessary that the cylindrical attachment 53 inserted in the tubular flange 52 be securely fixed to the flange 52. If the cylindrical attachment 53 and the tubular flange 52 were not securely fixed to each other, then the contents would leak through the gap between the cylindrical attachment 53 and the tubular flange 52. When the plug is removed from the opening 51, the cylindrical attachment 53 and the plug may possibly turn together, and the plug may not be detached from the cylindrical attachment 53. To alleviate this drawback, a seal member 55 is inserted between the tubular flange 52 and the cylindrical attachment 53 to allow the cylindrical attachment 53 to be reliably secured to the tubular flange 52 in intimate contact therewith. However, the process of inserting the seal member 55 is complex and costly.
The cylindrical attachment 53 that is inserted in the tubular flange 52 has a radially outwardly extending peripheral edge 56 projecting on the inner surface of the top plate 50, defining a stepped recess 57 thereon. When the stored material is removed from the metallic drum container, some material tends to be trapped in the recess 57, and the metallic drum container cannot fully be emptied. The trapped material cannot easily be cleared out of the recess 57 when the interior of the metallic drum container is cleaned for reuse.
U.S. Pat. No. 4,852,238 (Japanese Laid-Open Patent Publication No. 1-313119) discloses a method of making a top plate which has an opening but does not have any separate attachment.
According to the disclosed method, a region of the top plate where an opening is to be defined is raised upwardly to form a flat disk on its top, and the top plate stock is drawn from the center of the disk toward the peripheral edge thereof. Then, a circular hole that is smaller in diameter than the opening to be eventually formed is defined in the disk. The top plate stock that has been flowed toward the peripheral edge of the disk is raised upwardly into a tubular flange. At the same time, the circular hole is enlarged in diameter. Thereafter, the entire peripheral edge of the tip end of the flange is vertically compressed to increase the thickness of the flange. The inner wall surface of the flange is then internally threaded.
The increased thickness of the flange increases the mechanical strength of the flange. A plug is directly inserted in the tubular flange in threaded engagement with its internally threaded inner wall surface.
Since no separate attachment is employed, the cost is lowered, and the interior of the drum container can easily be cleaned.
However, the integral formation of the flange and the top plate poses the following problems
Top plates for use on drum containers are generally made of thin sheet steel having a thickness in the range of from 1.0 mm to 1.2 mm, specifically SPHC for general use according to JIS (Japanese Industrial Standard) of Hot-Rolled Miled Steel Sheet, Strip and Plate, or SPCC for general use according to JIS of Cold-Rolled Carbon Steel Sheet and Strip. The top plates are required to have an opening for threaded engagement with a 2-inch (50.8 mm) plug and an opening for threaded engagement with a 3/4-inch (19.05 mm) plug according to international standards. The flanges around the openings must have a height of about 8 mm so that the inner wall surfaces of the flanges are internally threaded over a length of 6 mm or more for threaded engagement with the plugs.
The SPHC, referred to above, whose wall thickness is 1.6 mm or less has an elongation percentage of 27%, which is smaller than the elongation percentage of 30% of SPHD that is to be machined by drawing and the elongation percentage of 31% of SPHE that is to be machined by deep drawing. Likewise, the SPCC whose wall thickness is 1.6 mm or less has an elongation percentage of 37%, which is smaller than the elongation percentage of 39% of SPCD that is to be machined by drawing and the elongation percentage of 41% of SPCE that is to be machined by deep drawing.
When a flange that is 8 mm high is formed on a top plate of SPHC or SPCC whose wall thickness ranges from 1.0 mm to 1.2 mm to define a 3/4-inch opening according to the above conventional process, the flange may crack under stresses because of the limited elongation percentage.
The inner wall surface of the flange should preferably be internally threaded by roll threading rather than cutting because cut threads would reduce the mechanical strength of the flange. If a small crack were formed in the tip end of the flange at the time it is drawn, it might develop into a larger crack when the flange is internally threaded, and the top plate could not be available as a final product. Even with no crack formed in the flange, if the flange were progressively thinner toward its upper edge, then the flange might crack when it is internally threaded. To avoid this shortcoming, after the flange is formed by deep drawing, it is downwardly compressed to prevent the upper edge of the flange from becoming thinner, according to the conventional method described above. However, the step of downwardly compressing the flange in addition to the step of forming the flange makes the manufacturing apparatus complex.